The present invention relates to substantially linear polyphosphazenes and to a process for their production.
Polyphosphazenes are inorganic macromolecules having backbones made up of alternating phosphorus and nitrogen atoms. The physical and chemical properties of polymeric phosphazenes may be varied by changing the substituents attached to the phosphorus atoms in the backbone. For example, Allcock discloses in "Developments at the Interface of Inorganic, Organic, and Polymer Chemistry", Chemical and Engineering News. Vol. 63, pages 22-36 (1985) that inclusion of methoxy, ethoxy and/or mixtures of groups such as CF.sub.3 CH.sub.2 O or HCF.sub.2 CF.sub.2 CH.sub.2 O groups as the substituents attached to phosphorus atoms in the backbone of the polyphosphazene produce polyphosphazenes having improved low temperature flexibility. Inclusion of fluoroalkoxy side groups on the backbone produce polyphosphazenes having surface hydrophobicity and solvent resistance. Most polyphosphazenes having such side groups are also said to resist burning and oxidative breakdown better than many single strand organic polymers and to be stable with respect to water and a wide range of other chemical agents.
There are two main approaches to making polyphosphazenes which are currently being used. In the first, halogenated cyclotriphosphazenes are first heated to open the ring and then polymerized. The halogen substituents are subsequently replaced by hydrolytically stable groups. Typical examples of this process are given in Allcock, et al, JACS, Vol. 87, pages 4216-4217 (1965); Allcock, Chemical Review, Volume 72, pages 315-356 (1972); and Allcock, et al, Macromolecules, Volume 13, pages 201-207 (1980).
In the second approach to making polyphosphazenes, substituted phosphinimines are condensed. Such condensation processes are disclosed by Wisian-Neilson, et al, Inorganic Chemistry, Volume 19, pages 1875-1878 (1980); Neilson, et al, J. Macromol. Sci-Chem., Volume A16(1), pages 425-439 (1981) and Neilson, et al, Macromolecules, Volume 20, pages 910-916 (1987).
The condensation of substituted phosphinimines provides a route to a variety of polyalkylphosphazenes and polyarylphosphazenes in addition to the known polyalkoxy-, polyaryloxy- and polyamino-phosphazenes. However, these condensation reactions involve long reaction times (e.g., 2-12 days) and high temperatures (typically 160.degree.-220.degree. C.) before the desired polymer is obtained. These thermally driven condensation reactions also provide little control over the product's ultimate molecular weight and molecular weight distribution. Additionally, it would be expected that the high reaction temperatures used in these processes would lead to undesirable side reactions.
Polyphosphazenes such as poly[bis(2,2,2-trifluoroethoxy)phosphazene] may be prepared by either the ring opening/halogen substitution method or by condensation of the phosphinimine obtained by reaction of trimethylsilylazide with tris(2,2,2-trifluoroethyl) phosphite. Flindt, et al discloses an appropriate procedure for the condensation reaction in Z. Anorg. Allg. Chem., Vol. 428, pages 204-208 (1977). The polymerization step in the Flindt, et al process was carried out without a catalyst. Consequently, polymerization required a reaction time of approximately 48 hours at a temperature of approximately 200.degree. C.
It has been demonstrated by Sennett, et al (See Sennett, et al, Macromolecules, Volume 19, pages 959-964 (1986) that Lewis acids such as boron trichloride catalyze the ring opening polymerization of cyclohalophosphazenes. Mujumdar, et al have also reported in Macromolecules, Vol. 23, pages 14-21 (1990) that protonic acids such as toluenesulfonic acid, sulfobenzoic acid and sulfamic acid catalyze the ring opening polymerization of cyclohalophosphazenes. However, no material which effectively catalyzes polymerization of phosphinimine monomers has been found in the literature.